The present invention relates to a passive flowmeter for determining the flow rate and the direction of flow of a fluid in a well.
In a producing well it is important to determine the mean flow rate of the fluid flowing therein as a function of depth. This information serves to locate the production zones. Also, it is important to determine the direction of fluid flow in the well simultaneously with the flow rate.
A conventional flowmeter used for logging production well generally comprises a support adapted to be lowered into the well at the end of a cable and a spinner rotatably mounted on the support. The speed of rotation of the spinner is a function of the flow rate of the fluid in the borehole. The direction of rotation of the spinner is a function of the direction of flow of the fluid in the borehole.
The flowmeter comprises a sensor for generating electrical signals representative of the speed and the direction of rotation of the spinner. These signals are electrical pulses whose period is a function of the speed of rotation. The pulses can be counted to provide the speed of rotation of the spinner. The signals are transmitted through the cable to the surface where the speed and the direction of rotation of the spinner are recorded as a function of depth.
U.S. Pat. No. 3,433,070 describes a device which generates electrical signals from which the flow rate and the direction of flow of a fluid are determined. This device is an active device. More precisely, an electrical generator provides a sinewave electrical signal to a coil. The coil is a portion of a resonant circuit and the frequency of the electrical signal is equal to the resonant frequency of the system.
The coil surrounds a fixed magnetic bar which is excentric relative to the axis of the coil. Variation in the tuning of the resonant circuit is measured while an asymmetrical magnetic member is rotated about the coil. The moving magnetic member is rotated by the spinner.
The changes in inductance of the coil during rotation of the moving member modulate the amplitude of the electrical signal flowing through the coil. This modulation is measured electrically in such a manner as to determine the flow rate and the direction of fluid flow.
U.S. Pat. No. 4,566,317 describes another active device generating signals which enable the flow rate and the direction of flow of a fluid to be deduced.
A metal cross is fixed to the spinner. Two fixed coils face the cross. Each of the coils is connected to a resonant circuit and is fed with a periodic electric current. The cross is rotated by the spinner and this changes the amplitude of the electrical current flowing through the coils since the impedance of each coil is modified by the proximity of the metal arms of the cross.
Each rotation of the cross generates current pulses at the outlet of each oscillating circuit and these pulses are processed electronically for the purpose of deducing the flow rate and the direction of flow of the fluid.
These two active devices suffer from the major drawback of requiring sources of electrical current which are perfectly stable for delivering current to be modulated by the rotation of the spinner.
U.S. Pat. No. 3,162,042 describes a passive device providing an electrical signal from which the flow rate and the direction of flow of the fluid can be deduced. An annular magnetic core supports three equidistant coils which are interconnected. In the central portion of the annular core, a magnetic element having a north pole and a south pole is rotated by the spinner. Current is thus generated in the coils and the desired information can be deduced therefrom after processing.
An object of the present invention is to provide a passive device delivering an asymmetric periodic signal which, after appropriate electronic processing, makes it possible to deduce the mean flow rate and the direction of flow of a fluid in a well.
The present invention makes it possible to perform such measurements with improved resolution, even when the fluid flow rate is low.